FIG. 1 is a drawing to outline handover that takes place in a mobile radio communication cellular system. In FIG. 1, mobile station 11 is currently carrying out radio communication with base station 31 installed in cell 21 and moving toward cell 22 adjacent to cell 21.
When mobile station 11 moves from cell 21 to cell 22, mobile station 11 carries out diversity handover between base station 31 and base station 32. During the diversity handover, mobile station 11 measures the reception quality of a common control channel by communicating with both base station 31 and base station 32. Then, when the reception quality of base station 32 becomes better than that of base station 31, mobile station 11 switches its communication counterpart from base station 31 to base station 32.
Furthermore, during the diversity handover, control station 41 selects and combines signals with good reception conditions from among signals sent from mobile station 11 and received by base station 31 and base station 32.
Here, in a cellular system, closed-loop transmit power control is performed between the mobile station and base stations to improve the frequency utilization efficiency by reducing interference with other communication stations. Transmit power control by the base stations is carried out by referring to the reception quality such as a reception SIR (signal to interference ratio) and the error rate of the signal sent from the mobile station, generating information to control the transmit power of the mobile station (Transmit-Power Control: hereinafter referred to as “TPC”) so that the reception quality satisfies a certain standard and sending the TPC to the mobile station.
Transmit power control using the reception SIR by the base stations is carried out by receiving an individual channel sent from the mobile station subject to the transmit power control and calculating the ratio of the power of the desired signal to the other reception power, that is, the power of the interference signal.
It is a general practice that an average power value over several slots or frames instead of an instantaneous value is used as the interference signal power value and this is disclosed in the Unexamined Japanese Patent Publication No.HEI 10-13364.
During diversity handover, transmit power control is performed simultaneously between mobile station 11 and base stations 31 and 32, and mobile station 11 acquires TPCs sent from base stations 31 and 32 and carries out transmit power control of the own station based on those TPCs.
Since the reception signals of base stations 31 and 32 are selected and combined by control station 41, not all base stations in communication need to satisfy the reception quality standard and mobile station 11 determines transmit power according to the TPC with minimum transmit signal power of a plurality of TPCs received.
However, when diversity handover starts, that is, when the mobile station starts communication with a new base station in addition to the base station currently in communication, the base station that newly enters into communication has no sufficiently observed individual channels, which are to be used for measurement of the SIR of the mobile station and to be averaged, and therefore averaging causes the power of the signal of interference with the mobile station to be measured lower than the actual value.
Therefore, the base station that has newly entered into communication determines that the reception SIR of the mobile station is satisfactory and transmits a TPC instructing the mobile station to reduce its transmit power.
Even if the mobile station receives from the base station currently in communication a TPC instructing that the transmit power should be increased, the mobile station receives from the base station that has newly entered into communication the TPC instructing that the transmit power should be reduced, and therefore the mobile station reduces transmit power according to the TPC instructing the minimum transmit signal power even if the reception SIR is actually not satisfactory for the base station currently in communication.
For this reason, the reception quality further deteriorates and satisfactory reception quality is not obtained from either of the base stations involved in the diversity handover, which prevents the control station from combining the reception signals. As a result, it will be necessary to retransmit the transmission unit, causing problems of reducing the communication efficiency and interrupting radio connections.